The base material for FPC is FCCL and the structures are Copper +PI, Copper + AD + PI, Copper + PI +Copper or Copper + AD + PI + AD + Copper.
Originally, copper is very key for the flex circuits material. Which kind of copper we could choose? The answer is ED Copper (Electrodeposited Copper Foil) and RA copper (Rolled Annealed Copper Foil). Then what the difference between these two? I will show it from below 4 aspects:
Different copper is defined by its manufacturing method. ED Copper, which is made from CuSO4 solution, by using electrolysis method, made Cu2+ dip into spinning cathode rolls and stripping, then made it into ED copper. RA Copper, which is made from high purity copper(>99.98%), by using pressure process, made copper into different copper thickness.
ED Copper (Electrodeposited Copper Foil) and RA Cooper (Rolled Annealed Copper Foil)
Due to different manufacturing methods and different shapes, the performance of them is not same. ED copper has better conductivity than RA copper. While as for extensibility, RA copper is much better than ED copper.
Because of different performances, RA copper and ED copper are used in different products which tend to different product requests. Normally, ED copper is always used for static application and RA copper is always used in dynamic application. For example, the flex board needs to be bendable a lot, so obviously, RA copper will be a good choice. Otherwise, we could use ED copper with high electrical conductivity. Whatever, you should select copper according to your circuits' requests (final board performance, cost, function...)